1. Field Of The Invention
This invention relates to measuring process parameters, and in particular, to measuring process parameters related to heat flux.
2. Background Art
Drying is often used to achieve stability of a material. The drying process may be as simple as the direct evaporation of water from a system or it may be a more complex process such as the lyophilization process. The lyophilization process involves freezing of the material, sublimation of the ice crystals, and desorption of the remaining water vapor.
Regardless of the type of drying process, there is a need to know when the material has been sufficiently dried. If insufficient water is removed from the system, there may be a loss in product or its stability. Likewise, the material may be damaged if the product is over-dried. Furthermore, many other parameters related to various methods of processing materials must be reliably measured in order for a material to be processed properly.
A number of methods are known in the prior art for monitoring various parameters involved in the drying process and other types of processes. The simplest method of monitoring a process is to measure the temperature of the material being processed. A change in temperature indicates the completion of a particular phase of the process. Other techniques are known for monitoring both the composition of the gas released during a process such as a drying process and the shelf temperature during the process. The change in the partial pressure of water vapor is also used as an indication of the completion of a drying process. Still other known prior art methods for monitoring the processing of materials such as the drying of the material involve measurement of an electrical property of the material. For example, the resistance of the material may be measured.
The use of these methods, either separately or in combination, provides at best a qualitative assessment of the process. They do not take into account variations such as changes in the quantity of solvent present in the material during the processing. For example, the quantity of water in a material varies as it is dried.
It is also known to use differential scanning calorimetry as an analytical tool. In this method, two containers are placed on separate heating surfaces to measure the heat into a sample container and the heat into a reference container when the two containers are placed on the separate heating surfaces of the calorimeter. These differential scanning calorimetry systems are complex because of the problems raised by the use of the two separate heating surfaces. Further complicating the use of these calorimetry systems is the practical consideration that no two containers are exactly alike in their thermal properties. Thus, the two separate heating surfaces for heating the reference container and the sample container of the differential scanning calorimetry system must be run at different temperatures to compensate for the different thermal properties of the containers. This problem made calibrating differential scanning calorimetry systems very complex.
A further drawback in the use of known differential scanning calorimetry techniques is that it is not possible to determine the drying rate of a material during the monitored process. Differential scanning calorimetry can be used to determine parameters of a material prior to processing of the material. Additionally, further determinations of the parameters of materials can be made after processing of the material is complete. However, the drying rate can not be determined from this data.